1. Field of the Invention
The present invention relates to a waveguide type photoreceptor device, and more particularly to a waveguide type photoreceptor device used for optical communications systems, etc.
2. Description of the Related Art
The capacity of communications systems has been increased to satisfy the dramatically increasing demand for communications. Accordingly, there has been a need for higher-speed, higher-efficiency yet lower-cost and smaller optical communications devices.
The transmission systems for optical communications use two types of signal light having different wavelength bands (or signal light with two wavelength bands). One is a 1.3 μm (wavelength) band signal light whose center wavelength is 1.3 μm, and the other is a 1.55 μm (wavelength) band signal whose center wavelength is 1.55 μm.
The 1.55 μm band signal light causes only a small optical fiber loss and therefore is used for long-distance communication systems. This type of communication is referred to as the intercity communication (trunk system) and used for communications between large cities such as Tokyo and Osaka.
The 1.3 μm band signal light, on the other hand, causes a large optical fiber loss but exhibits a low wavelength dispersion level and therefore is used for short-distance communication systems. This type of communication is referred to as the intracity communication and used for communications within large cities. The 1.3 μm band signal light is also used for communications between each base station and homes. Such a system is referred to as an access system.
To receive these two types of signal light having the different wavelength bands (or signal light with the two wavelength bands), optical communication systems have used two waveguide type semiconductor photodiodes each adapted for signal light with one of the wavelength bands.
A well-known example of a conventional waveguide type photoreceptor device is configured such that an n conductive type InGaAsP light guide layer, an intrinsic InGaAs light absorption layer, a p conductive type InGaAsP light guide layer, and a p conductive type InP cladding layer are sequentially laminated onto one another over an n conductive type InP substrate (n conductive type, p conductive type, and intrinsic semiconductor are hereinafter expressed as “n-n”, “p-”, and “i-”, respectively). The n-InGaAsP light guide layer has a thickness of 1.7 μm and a bandgap wavelength of 11.3 μm, while the p-InGaAsP light guide layer has a thickness of 0.3 μm and a bandgap wavelength of 1.3 μm (see, for example, paragraphs [0024] to [0026] and FIG. 1 of Japanese Laid-Open Patent Publication No. 2001-24211).
Another well-known example (a waveguide type semiconductor photoreceptor device used for optical communications systems) is configured such that a waveguide mesa made up of an n-InP cladding layer, an n+-InAlGaAs guide layer, an i-InGaAs light absorption layer, a p+-InAlGaAs guide layer, a p+-InP cladding layer, and a p+-InGaAs contact layer is formed on a semi-insulative InP substrate. The n+-InAlGaAs guide layer has a layer thickness of 0.8 μm, the i-InGaAs light absorption layer has a layer thickness of 0.5 μm, and the p+-InAlGaAs guide layer has a layer thickness of 0.1 μm (see, for example, paragraph [0003] and FIG. 13 of Japanese Laid-Open Patent Publication No. 2002-203984).
Still another well-known example (a 1.5-μm band 10-Gb/s waveguide type PIN-PD used for optical communications networks having a communication capacity on the order of gigabits or more) is of a mesa type having an InGaAlAs double core structure and includes a light absorption layer of In0.53Ga0.47As. See, for example, “Characteristics of 1.5-μm Band 10-Gb/s Waveguide Type PIN-PD”, Manuscript for the 50th Lecture Meeting of the Japan Society of Applied Physics, Kanagawa University, pp. 1246, 27p-H-15, Spring 2003.
Conventional waveguide type photoreceptor devices are each configured of a photodiode adapted for signal light with a single wavelength band used by a target optical communications system. With an increase in the amount of transmission in optical communications systems, however, a communications network currently established for intracity communication may also be used for intercity communication. In such a case, the above conventional arrangement in which optical components (such as photoreceptor devices) are adapted only for a single wavelength complicates the configuration of each communication device in optical communications systems.
Furthermore, optical components such as waveguide type photodiodes (hereinafter referred to as waveguide type PDs) adapted for signal light with a single wavelength have been difficult to operate at high speed with high sensitivity when they receive signal light with two wavelengths.
A waveguide type PD has a structure in which light is confined within the waveguide portion made up of a light absorption layer and light guide layers sandwiching the light absorption layer, and the light confined within the waveguide portion is absorbed and converted into an electric signal while the light is propagating through the light guide layers and the light absorption layer.
Since the waveguide type PD confines light within its waveguide portion and absorbs it by utilizing the differences between the refractive indices of the light absorption layer, the light guide layers, and the cladding layer, the appropriate refractive index of each layer varies depending on the wavelength of the signal light which the waveguide type PD is designed to receive.
The device structure of a waveguide type PD for a single wavelength band can be optimized according to the wavelength band of the light to be received. A waveguide type PD for more than one wavelength, however, may have a problem in that it may have good sensitivity characteristics at one wavelength but have very bad sensitivity characteristics at another wavelength. It may even happen that the waveguide type PD has undesirable sensitivity characteristics over the entire wavelength band.
For example, since increasing the differences between the refractive indices of the light guide layers and the cladding layers increases the amount of light confined within the waveguide, it may be a good idea to set the light guide layers such that they have as long a composition wavelength as possible selected from among those at which the bandgap signal light can transmit through the light guide layers.
To accommodate more than one wavelength, however, the light guide layers must have a composition wavelength at which signal light with the shortest wavelength band can transmit through them.
If the composition wavelength of the light guide layers is determined based on a wavelength in the shortest wavelength band of the signal light, the sensitivity of the waveguide type PD for the other wavelength bands may considerably degrade.
If the n-light guide layer and the p-light guide layer sandwiching the light absorption layer have the same layer thickness (that is, these guide layers are symmetrical to each other about the light absorption layer), the mode of the light propagating within the waveguide stabilizes and thereby the amount of light propagating through the light guide layers increases, causing the problem of reduced photoelectric conversion efficiency. To solve this problem, the light guide layers may be set to have different layer thicknesses (they may be set asymmetrical to each other about the light absorption layer). Even with such a waveguide structure in which the light guide layers are asymmetrical to each other about the light absorption layer, however, a waveguide type PD for more than one wavelength may have very bad sensitivity characteristics at some wavelength through it may have good sensitivity characteristics at a different wavelength. Furthermore, with a simple asymmetrical waveguide structure, the waveguide type PD may have degraded sensitivity characteristics even for signal light with a single wavelength band in some cases.
Thus, it is difficult to form a waveguide type PD having a waveguide structure in which the light guide layers are asymmetrical to each other about the light absorption layer in such a way that the waveguide type PD can operate at high speed with high sensitivity for both (signal light with) a first wavelength band and (signal light with) a second wavelength band (or another wavelength band) at the same time. In some cases, such a waveguide type PD is difficult to operate at high speed with high sensitivity even when it receives signal light with a single wavelength.